This invention relates to the field of electroluminescent lamps; more particularly, the invention relates to a power factor compensating electroluminescent DC to AC inverter circuit.
In its most basic form, an electroluminescent lamp or cell contains a layer of electroluminescent phosphor compound sandwiched between a pair of conductive electrode coatings or layers at least one of which is transparent. In the electroluminescent cell art, a plurality of additional layers for various purposes can be added to this basic configuration. From an electrical point of view, the electroluminescent electrically active cell comprises two electrodes sandwiching the electroluminescent phosphor and behaves like a capacitor.
U.S. Pat. No. 3,173,057--Electroluminescent Combination by W. A. Thorton, Jr.--recognizes that electroluminescent lamps display a capacitance which decreases as the device is operated, thereby decreasing the capacitive reactance of the device. Thornton in recognizing this discloses a circuit in which the electroluminescent cell and the inductor are chosen with respect to one another so that they form a series resonant circuit. This series resonant circuit is actuated by a potential source which is adopted to deliver a predetermined potential of a predetermined frequency. When the device is initially operated, the components of the series resonant circuit have such relative values of resistance and reactance that the potential developed across the inductor differs by a predetermined amount from the potential developed across the electroluminescent device. During operation, the normal decrease in the light output of the electroluminescent device portion of the series resonant circuit is modified by the change in the effective energizing potential which is developed across the electroluminescent device, as the operation of the series resonant circuit is shifted with respect to a condition of resonance because of the decrease in capacitance of the electroluminescent device.
U.S. Pat. No. 3,749,977--Electroluminescent Device by Sliker--also recognizes the capacitive properties of electroluminescent lamps. Oscillator circuits for use with alternating current sources are shown which result in reduced power requirements. This circuit is for use with an alternating current source. The lamp load is in series with the primary transformer winding of the oscillator circuit.
Electrical ballast circuits are known to be used to improve the power factor of gaseous discharge lamps. Generally, these circuits use a combination of inductors and capacitors. In U.S. Pat. No. 4,123,690--Discharge Lamp Ballast Circuit by Osteen, an electric ballast circuit of improved power factor for operating gas discharge lamps is disclosed. Two discharge lamps are respectively connected to two inductively coupled induction coils for obtaining high power factor without the use of capacitors. By virtue of the circuit arrangement, the current supplied to the two lamps is substantially out of phase and an improvement in power factor is obtained by virtue of the phase difference between these currents. The current through one lamp leading the current through the second lamp serves, in effect, as a capacitor to the line.
U.S. Pat. No. 4,145,636--Fluorescent Lamp Driving Circuit by Doi--discloses a circuit for driving a fluorescent lamp with a direct current source. The circuit includes a transistor and a transformer, the secondary coil of which is divided into a feedback portion and a second portion containing the lamp load. One filament of the lamp serves as part of the bias resistance in series with the feedback portion and at the same time is heated by the base current flowing through the base emitter-junction of the transistor.
U.S. Pat. No. 3,821,635--Capacitor Charging Circuit by Kimmel et al, although not related to lamp loads, discloses a power factor control circuit associated with the power supply having a solid state switch in series with the primary transformer winding, a feedback circuit for the control gate of the solid state switch extending from the load connecting the secondary winding of the transformer. In operation, the power factor control circuit reflects back to the current regulation circuit output, the impedance of the capacitor as it is charging and is responsive to a predetermined condition thereof for changing the apparent resistance reflected, in order to maximize the circuit power factor. The purpose of this circuit is the repeatedly charging and discharging of the capacitor for use in a flashtube apparatus or the like. It effectively changes AC to DC.
U.S. Pat. No. 4,068,158--High Efficiency DC Voltage Regulator by Hickes et al, discloses a high efficiency DC voltage regulator in which there is a transformer with a primary winding in series with a switching transistor which is provided with a low impedance source of bias current. The bias current for the transistor is derived by storing energy supplied by the secondary winding of the transformer with a capacitor or other energy storage means during a nonconducting portion of this switching cycle. The stored energy is transferred to the base of the transistor in the form of a bias current. This additional bias current reduces the current drawn through the bias resistors thereby reducing the energy consumption in the bias resistors.
Electroluminescent lamps are used with alternating current. Certain applications, such as in motor vehicles and aircraft, require that electroluminescent lamps be used where only a direct current is available. In these applications, inverters that are generally known in the art can be used to translate the direct current to an alternating current. The unique properties of electroluminescent lamps make them very inefficient when used with these conventional inverters. For one thing, electroluminescent lamps are highly capacitive, resulting in power factors as low as about 0.15 to about 0.4. Another problem with electroluminescent lamps is that there is an exponential decay in light output. With the decay in light output, there is an additional lowering in the power factor. Therefore, we are dealing with an element in the circuit having varying properties and which further causes a low power factor.
Most low cost transistor inverters have the transistor base drive provided by a special feedback winding with a resistance network, an R-C network, or resistance-capacitor network, or in some cases by a simple resistor fill up tied to the collector of the complementary transistor of a simple push-pull inverter.
None of these low cost inverters have inherent power factor correction compensation. Their use then might be expected to result in higher power input requirements and lower efficiencies with the switching transistor device usually taking the brunt of the losses. The prior art teaches the use of power factor control devices particularly for regulating incoming alternating current. Related are the power factor correction circuits used with gas discharge lamps. However, there is no art in the area of power compensating DC to AC inverters for use with electroluminescent lamps.
Therefore, there is a need for a circuit in which direct current is inverted with power factor correction and a means to compensate for the changing properties of the device such as an electroluminescent lamp element within the circuit.